1. Field of Invention
This invention relates to an electromagnetic clutch assembly, such as for use in controlling the transmission of power from an automobile engine to the refrigerant compressor in an automotive air-conditioning system and, more particularly, to an electromagnetic clutch assembly that includes an apparatus for interrupting the transmission of the driving force from an automobile engine to the compressor should the compressor become locked.
2. Description of the Prior Art
An automotive air-conditioning system typically includes a refrigerant circuit having a refrigerant compressor, a condenser, an expansion valve and an evaporator connected in series. The automobile engine provides the driving force for the compressor via a transmitting device, such as a driving belt, which is connected to the respective pulleys or rotors associated with the automobile engine and the compressor.
Generally, a rotor associated with the compressor forms a part of an electromagnetic clutch which can intermittently transmit the driving force from the automobile engine to the compressor by virtue of the intermittent energization of an electromagnetic coil of the electromagnetic clutch. The electromagnetic coil of the clutch intermittently energizes in response to a thermodynamic characteristic of the refrigerant circuit, such as the temperature of air leaving from the evaporator. The intermittent energization of the electromagnetic coil controls the intermittent operation of the compressor. Thereby, temperature in a passenger compartment of the automobile can be maintained at a certain value.
In addition, many devices utilize the driving force of an automobile engine, including automobile generators, oil hydraulic pumps for power assisted steering wheels and power assisted brakes, and refrigerant compressors. In recent years, in order to effectively utilize the restricted vacant space in an automobile engine compartment, the respective pulleys or rotors associated with the automobile engine and the devices driven by the engine are arranged to use only one driving belt.
Though space efficient, the above-described arrangement is extremely disadvantageous, especially if, for example, the driven device of the compressor, i.e., the drive shaft, should stop its rotational motion due to unexpected malfunctioning or trouble with the compressor's parts. Should the drive shaft stop while the rotor of the compressor is frictionally engaged with the armature plate (which is also connected to the drive shaft), the contacting surfaces of the rotor and armature plate will slide relative to each other, creating intense frictional heat. This heat will be conducted to a bearing which rotatably supports the drive shaft, and may cause seizure thereof. The seizure of the bearing intensifies the frictional heat generation of the rotor and armature plate. Consequently, the driving belt, unable to endure intensified heat, will burn and tear. As a result, the other devices utilizing the engine power transmitted by the driving belt will be rendered inoperative as well.
In order to overcome above-mentioned defect, Japanese Utility Model Application Publication No. 57-25222 discloses an electromagnetic clutch including a device which can immediately release the engagement between the rotor and the armature plate should the compressor become locked. This prior art clutch assembly for an automotive air-conditioning system is illustrated in FIG. 1.
Referring to FIG. 1, compressor housing 10 is provided with tubular extension 11 which surrounds drive shaft 20 of the compressor. Drive shaft 20 is rotatably supported in compressor housing 10 by a bearing (not shown).
Rotor 30 is rotatably supported on tubular extension 11 through bearing 12 which is mounted on an outer surface of tubular extension 11. Rotor 30 is made of magnetic material, such as steel, and comprises outer annular cylindrical portion 31, inner annular cylindrical portion 32 and axial end plate portion 33 connecting outer and inner cylindrical portion 31, 32 at one end. Thus, annular cavity 34 is defined by portions 31, 32 and 33. Annular V-belt groove 311 is formed on an outer peripheral surface of outer cylindrical portion 31 for receiving V-belt 40, which couples the compressor to the engine of the automobile (not shown). Axial end plate portion 33 has a frictional surface 331 formed on its outer surface and includes one or more concentric slits 332 to define a plurality of annular or arcurate pieces.
Annular housing 51 having a substantially U-shaped cross section is disposed in annular cavity 34 of rotor 30. Electromagnetic coil 50 is contained within annular housing 51. Housing 51 is fixed to supporting plate 52, which is secured to an axial end surface of compressor housing 10 by a plurality of rivets (not shown). Annular housing 51 is thus maintained within cavity 34 without contacting rotor 30. Thermal detecting element 54, including a fusible metal, is disposed in an end portion of annular cavity 510 defined by annular housing 51 so as to be adjacent to axial end plate portion 33 of rotor 30. Cavity 510 is filled with heated epoxy resin 511, which has been hardened by elapsing time and cooling, so as to fixedly and insulatedly dispose coil 50 and thermal detecting element 54 therewithin. Thermal detecting element 54 is connected in series with electromagnetic coil 50 between a battery (not shown) and ground potential. A first terminal end of thermal detecting element 54 is connected to the battery, and a second terminal end of thermal detecting element 54 is connected to a first terminal end of coil 50. A second terminal end of coil 50 is connected to a first terminal end of wire 53 which is led from a bottom end portion of annular housing 51. A second terminal end 53a of wire 53 is connected to one terminal end of another wire (not shown) led from a control apparatus (not shown) of the automotive air-conditioning system.
Hub 60 is disposed on a terminal end of drive shaft 20 and is secured to drive shaft 20 by nut 21 and key 22. Hub 60 is provided with flange portion 61 extending radially outwardly from an axial end portion of hub 60. Annular armature plate 70 is concentrically disposed around an outer surface of hub 60 so as to face axoal plate portion 33 of rotor 30 with a predetermined axial air gap. Armature plate 70 also comprises frictional surface 71 which faces frictional surface 331 of rotor 30. Armature plate 70 is provided with one or more slits 72 to define a plurality of annular or arcuate magnetic pieces. Armature plate 70 is elastically connected to flange portion 61 of hub 60 through a plurality of flexible leaf springs 80. One end portion of each leaf spring 80 is secured on the outer end surface of armature plate 70 by rivet 73. The other end portion of leaf spring 80 is secured on an axial end surface of flange portion 61 of hub 60 by rivet 611. Stopper plate 90 and washer 612 are also fixed on flange portion 61 of hub 60 by rivet 611.
In the above-described electromagnetic clutch, if coil 50 is not energized, armature plate 70 is biased away from rotor 30 by the recoil strength of leaf springs 80. When electromagnetic coil 50 is energized, a magnetic flux is induced and flows through a closed loop comprising coil 50, housing 51, rotor 30 and armature plate 70. Armature plate 70 is thus magnetically attracted to frictional surface 331 of rotor 30 and springs 80 are flexed in the axial direction. In this manner, armature plate 70 moves axially so that frictional surface 71 engages frictional surface 331. This engagement transmits the engine-driven rotation of rotor 30 through armature plate 70, leaf springs 80 and hub 60 to drive shaft 20 of the compressor.
If the compressor should become locked while armature plate 70 is contact with rotor 30, frictional surface 331 of axial end plate portion 33 of rotor 30 will slide on frictional surface 71 of armature plate 70. Intensive frictional heat is caused by this sliding frictional contact. Thus, rotor 30 and armature plate 70 are heated rapidly. If the frictional heat created by the frictional surfaces between armature plate 70 and rotor 30 exceeds the melting point of the fusible metal of thermal detecting element 54, the fusible metal will melt. Thereby, the flow of current from the battery to coil 50 is disconnected so that coil 50 is deenergized. Accordingly, armature plate 70 separates from frictional surface 331 of axial end plate portion 33 of rotor 30 by the recoil strength of leaf springs 80. Therefore, rotor 30 idly rotates and V-belt 40 is protected from damage. Thus, other devices which utilize the driving force of the automobile engine can remain operable, even if the refrigerant compressor becomes locked.
However, the above-describe prior art solution presents further disadvantages. Since thermal detecting element 54 is disposed in the right end portion of annular cavity 510 of annular housing 51, the size of coil 50 must be reduced in relation to the volume of cavity 510, thereby decreasing the magnetic attraction force which acts between rotor 30 and armature plate 70. Furthermore, thermal detecting element 54 may fail due to the breaking of the fusible metal caused by the intensive vibration which propagates to, or is generated at, the electromagnetic clutch.